Fix dependency for Gotham nightly

When installing the Gotham nightly build over an existing Frodo installation - via `apt-get install xbian-package-xbmc-gotham-nightly` - the following is produced:

```
dpkg: xbian-package-xbmc: dependency problems, but removing anyway as you requested:
 xbian-package-upstart-xbmc-bridge depends on xbian-package-xbmc.
 xbian-update depends on xbian-package-xbmc | xbian-package-xbmc-6q; however:
  Package xbian-package-xbmc is to be removed.
  Package xbian-package-xbmc-6q is not installed.
```

I'm not sure if xbian-package-xbmc-6q still exists, as I can't find it in the repository, however I didn't remove it.